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rfc:rfc7496

Internet Engineering Task Force (IETF) M. Tuexen Request for Comments: 7496 Muenster Univ. of Appl. Sciences Category: Standards Track R. Seggelmann ISSN: 2070-1721 Metafinanz Informationssysteme GmbH

                                                            R. Stewart
                                                         Netflix, Inc.
                                                             S. Loreto
                                                              Ericsson
                                                            April 2015
           Additional Policies for the Partially Reliable
           Stream Control Transmission Protocol Extension

Abstract

 This document defines two additional policies for the Partially
 Reliable Stream Control Transmission Protocol (PR-SCTP) extension.
 These policies allow limitation of the number of retransmissions and
 prioritization of user messages for more efficient usage of the send
 buffer.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7496.

Tuexen, et al. Standards Track [Page 1] RFC 7496 Additional PR-SCTP Policies April 2015

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Additional PR-SCTP Policies . . . . . . . . . . . . . . . . .   3
   3.1.  Limited Retransmissions Policy  . . . . . . . . . . . . .   3
   3.2.  Priority Policy . . . . . . . . . . . . . . . . . . . . .   4
 4.  Socket API Considerations . . . . . . . . . . . . . . . . . .   4
   4.1.  Data Types  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.2.  Support for Added PR-SCTP Policies  . . . . . . . . . . .   5
   4.3.  Socket Option for Getting the Stream-Specific PR-SCTP
         Status (SCTP_PR_STREAM_STATUS)  . . . . . . . . . . . . .   6
   4.4.  Socket Option for Getting the Association-Specific
         PR-SCTP Status (SCTP_PR_ASSOC_STATUS) . . . . . . . . . .   7
   4.5.  Socket Option for Getting and Setting the PR-SCTP Support
         (SCTP_PR_SUPPORTED) . . . . . . . . . . . . . . . . . . .   8
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
   6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

Tuexen, et al. Standards Track [Page 2] RFC 7496 Additional PR-SCTP Policies April 2015

1. Introduction

 The Partially Reliable SCTP (PR-SCTP) extension defined in [RFC3758]
 provides a generic method for senders to abandon user messages.  The
 decision to abandon a user message is sender side only, and the exact
 condition is called a "PR-SCTP policy" ([RFC3758] refers to them as
 "PR-SCTP Services").  [RFC3758] also defines one particular PR-SCTP
 policy, called "Timed Reliability".  This allows the sender to
 specify a timeout for a user message after which the SCTP stack
 abandons the user message.
 This document specifies the following two additional PR-SCTP
 policies:
 Limited Retransmission Policy:  Allows limitation of the number of
    retransmissions.
 Priority Policy:  Allows removal of lower-priority messages if space
    for higher-priority messages is needed in the send buffer.

2. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

3. Additional PR-SCTP Policies

 This section defines two new PR-SCTP policies, one in each
 subsection.
 Please note that it is REQUIRED to implement [RFC3758], if you want
 to implement these additional policies.  However, these additional
 policies are OPTIONAL when implementing [RFC3758].

3.1. Limited Retransmissions Policy

 Using the Limited Retransmission Policy allows the sender of a user
 message to specify an upper limit for the number of retransmissions
 for each DATA chunk of the given user messages.  The sender MUST
 abandon a user message if the number of retransmissions of any of the
 DATA chunks of the user message would exceed the provided limit.  The
 sender MUST perform all other actions required for processing the
 retransmission event, such as adapting the congestion window and the
 retransmission timeout.  Please note that the number of
 retransmissions includes both fast and timer-based retransmissions.

Tuexen, et al. Standards Track [Page 3] RFC 7496 Additional PR-SCTP Policies April 2015

 The sender MAY limit the number of retransmissions to 0.  This will
 result in abandoning the message when it would get retransmitted for
 the first time.  The use of this setting provides a service similar
 to UDP, which also does not perform any retransmissions.
 Please note that using this policy does not affect the handling of
 the thresholds 'Association.Max.Retrans' and 'Path.Max.Retrans' as
 specified in Section 8 of [RFC4960].
 The WebRTC protocol stack (see [DATA-CHAN]) is an example of where
 the Limited Retransmissions Policy is used.

3.2. Priority Policy

 Using the Priority Policy allows the sender of a user message to
 specify a priority.  When storing a user message in the send buffer
 while there is not enough available space, the SCTP stack at the
 sender side MAY abandon other user message(s) of the same SCTP
 association (with the same or a different stream) with a priority
 lower than the provided one.  User messages sent reliably are
 considered to have a priority higher than all messages sent with the
 Priority Policy.  The algorithm for selecting the message(s) being
 abandoned is implementation specific.
 After lower-priority messages have been abandoned, high-priority
 messages can be transferred without the send call blocking (if used
 in blocking mode) or the send call failing (if used in non-blocking
 mode).
 The IP Flow Information Export (IPFIX) protocol stack (see [RFC7011])
 is an example of where the Priority Policy can be used.  Template
 records would be sent with full reliability, while flow records
 related to billing, security, and other monitoring would be sent
 using the Priority Policy with varying priority.  The priority of
 security-related flow records would be set higher than the priority
 of monitoring-related flow records.

4. Socket API Considerations

 This section describes how the socket API defined in [RFC6458] is
 extended to support the newly defined PR-SCTP policies, to provide
 some statistical information, and to control the negotiation of the
 PR-SCTP extension during the SCTP association setup.
 Please note that this section is informational only.

Tuexen, et al. Standards Track [Page 4] RFC 7496 Additional PR-SCTP Policies April 2015

4.1. Data Types

 This section uses data types from [IEEE.1003-1G.1997]: uintN_t means
 an unsigned integer of exactly N bits (e.g., uint16_t).  This is the
 same as in [RFC6458].

4.2. Support for Added PR-SCTP Policies

 As defined in [RFC6458], the PR-SCTP policy is specified and
 configured by using the following sctp_prinfo structure:
 struct sctp_prinfo {
   uint16_t pr_policy;
   uint32_t pr_value;
 };
 When the Limited Retransmission Policy described in Section 3.1 is
 used, pr_policy has the value SCTP_PR_SCTP_RTX and the number of
 retransmissions is given in pr_value.
 When using the Priority Policy described in Section 3.2, pr_policy
 has the value SCTP_PR_SCTP_PRIO.  The priority is given in pr_value.
 The value of zero is the highest priority, and larger numbers in
 pr_value denote lower priorities.
 The following table summarizes the possible parameter settings
 defined in [RFC6458] and this document:
   +-------------------+---------------------------+---------------+
   | pr_policy         | pr_value                  | Specification |
   +-------------------+---------------------------+---------------+
   | SCTP_PR_SCTP_NONE | Ignored                   | [RFC6458]     |
   | SCTP_PR_SCTP_TTL  | Lifetime in ms            | [RFC6458]     |
   | SCTP_PR_SCTP_RTX  | Number of retransmissions | Section 3.1   |
   | SCTP_PR_SCTP_PRIO | Priority                  | Section 3.2   |
   +-------------------+---------------------------+---------------+

Tuexen, et al. Standards Track [Page 5] RFC 7496 Additional PR-SCTP Policies April 2015

4.3. Socket Option for Getting the Stream-Specific PR-SCTP Status

    (SCTP_PR_STREAM_STATUS)
 This socket option uses IPPROTO_SCTP as its level and
 SCTP_PR_STREAM_STATUS as its name.  It can only be used with
 getsockopt() but not with setsockopt().  The socket option value uses
 the following structure:
 struct sctp_prstatus {
   sctp_assoc_t sprstat_assoc_id;
   uint16_t sprstat_sid;
   uint16_t sprstat_policy;
   uint64_t sprstat_abandoned_unsent;
   uint64_t sprstat_abandoned_sent;
 };
 sprstat_assoc_id:  This parameter is ignored for one-to-one style
    sockets.  For one-to-many style sockets, this parameter indicates
    for which association the user wants the information.  It is an
    error to use SCTP_{CURRENT|ALL|FUTURE}_ASSOC in sprstat_assoc_id.
 sprstat_sid:  This parameter indicates for which outgoing SCTP stream
    the user wants the information.
 sprstat_policy:  This parameter indicates for which PR-SCTP policy
    the user wants the information.  It is an error to use
    SCTP_PR_SCTP_NONE in sprstat_policy.  If SCTP_PR_SCTP_ALL is used,
    the counters provided are aggregated over all supported policies.
 sprstat_abandoned_unsent:  The number of user messages that have been
    abandoned using the policy specified in sprstat_policy on the
    stream specified in sprstat_sid for the association specified by
    sprstat_assoc_id, before any part of the user message could be
    sent.
 sprstat_abandoned_sent:  The number of user messages that have been
    abandoned using the policy specified in sprstat_policy on the
    stream specified in sprstat_sid for the association specified by
    sprstat_assoc_id, after a part of the user message has been sent.
 There are separate counters for unsent and sent user messages because
 the SCTP_SEND_FAILED_EVENT supports a similar differentiation.
 Please note that an abandoned large user message requiring SCTP-level
 fragmentation is reported in the sprstat_abandoned_sent counter as
 soon as at least one fragment of it has been sent.  Therefore, each
 abandoned user message is counted in either sprstat_abandoned_unsent
 or sprstat_abandoned_sent.

Tuexen, et al. Standards Track [Page 6] RFC 7496 Additional PR-SCTP Policies April 2015

 If more detailed information about abandoned user messages is
 required, the subscription to the SCTP_SEND_FAILED_EVENT is
 recommended.  Please note that some implementations might choose not
 to support this option, since it increases the resources needed for
 an outgoing SCTP stream.  For the same reasons, some implementations
 might only support using SCTP_PR_SCTP_ALL in sprstat_policy.
 sctp_opt_info() needs to be extended to support
 SCTP_PR_STREAM_STATUS.

4.4. Socket Option for Getting the Association-Specific PR-SCTP Status

    (SCTP_PR_ASSOC_STATUS)
 This socket option uses IPPROTO_SCTP as its level and
 SCTP_PR_ASSOC_STATUS as its name.  It can only be used with
 getsockopt(), but not with setsockopt().  The socket option value
 uses the same structure as described in Section 4.3:
 struct sctp_prstatus {
   sctp_assoc_t sprstat_assoc_id;
   uint16_t sprstat_sid;
   uint16_t sprstat_policy;
   uint64_t sprstat_abandoned_unsent;
   uint64_t sprstat_abandoned_sent;
 };
 sprstat_assoc_id:  This parameter is ignored for one-to-one style
    sockets.  For one-to-many style sockets, this parameter indicates
    for which association the user wants the information.  It is an
    error to use SCTP_{CURRENT|ALL|FUTURE}_ASSOC in sprstat_assoc_id.
 sprstat_sid:  This parameter is ignored.
 sprstat_policy:  This parameter indicates for which PR-SCTP policy
    the user wants the information.  It is an error to use
    SCTP_PR_SCTP_NONE in sprstat_policy.  If SCTP_PR_SCTP_ALL is used,
    the counters provided are aggregated over all supported policies.
 sprstat_abandoned_unsent:  The number of user messages that have been
    abandoned using the policy specified in sprstat_policy for the
    association specified by sprstat_assoc_id, before any part of the
    user message could be sent.
 sprstat_abandoned_sent:  The number of user messages that have been
    abandoned using the policy specified in sprstat_policy for the
    association specified by sprstat_assoc_id, after a part of the
    user message has been sent.

Tuexen, et al. Standards Track [Page 7] RFC 7496 Additional PR-SCTP Policies April 2015

 There are separate counters for unsent and sent user messages because
 the SCTP_SEND_FAILED_EVENT supports a similar differentiation.
 Please note that an abandoned large user message requiring SCTP-level
 fragmentation is reported in the sprstat_abandoned_sent counter as
 soon as at least one fragment of it has been sent.  Therefore, each
 abandoned user message is counted in either sprstat_abandoned_unsent
 or sprstat_abandoned_sent.
 If more detailed information about abandoned user messages is
 required, the usage of the option described in Section 4.3 or the
 subscription to the SCTP_SEND_FAILED_EVENT is recommended.
 sctp_opt_info() needs to be extended to support SCTP_PR_ASSOC_STATUS.

4.5. Socket Option for Getting and Setting the PR-SCTP Support

    (SCTP_PR_SUPPORTED)
 This socket option allows the enabling or disabling of the
 negotiation of PR-SCTP support for future associations.  For existing
 associations, it allows one to query whether or not PR-SCTP support
 was negotiated on a particular association.
 Whether or not PR-SCTP is enabled by default is implementation
 specific.
 This socket option uses IPPROTO_SCTP as its level and
 SCTP_PR_SUPPORTED as its name.  It can be used with getsockopt() and
 setsockopt().  The socket option value uses the following structure
 defined in [RFC6458]:
 struct sctp_assoc_value {
   sctp_assoc_t assoc_id;
   uint32_t assoc_value;
 };
 assoc_id:  This parameter is ignored for one-to-one style sockets.
    For one-to-many style sockets, this parameter indicates upon which
    association the user is performing an action.  The special
    sctp_assoc_t SCTP_FUTURE_ASSOC can also be used; it is an error to
    use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
 assoc_value:  A non-zero value encodes the enabling of PR-SCTP,
    whereas a value of 0 encodes the disabling of PR-SCTP.
 sctp_opt_info() needs to be extended to support SCTP_PR_SUPPORTED.

Tuexen, et al. Standards Track [Page 8] RFC 7496 Additional PR-SCTP Policies April 2015

5. Security Considerations

 This document does not add any security considerations to those given
 in [RFC4960], [RFC3758], and [RFC6458].  As indicated in the Security
 Considerations of [RFC3758], transport-layer security in the form of
 TLS over SCTP (see [RFC3436]) can't be used for PR-SCTP.  However,
 DTLS over SCTP (see [RFC6083]) could be used instead.  If DTLS over
 SCTP as specified in [RFC6083] is used, the Security Considerations
 of [RFC6083] do apply.  It should also be noted that using PR-SCTP
 for an SCTP association doesn't allow that association to behave more
 aggressively than an SCTP association not using PR-SCTP.

6. References

6.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
            Conrad, "Stream Control Transmission Protocol (SCTP)
            Partial Reliability Extension", RFC 3758, May 2004,
            <http://www.rfc-editor.org/info/rfc3758>.
 [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
            RFC 4960, September 2007,
            <http://www.rfc-editor.org/info/rfc4960>.

6.2. Informative References

 [RFC3436]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
            Layer Security over Stream Control Transmission Protocol",
            RFC 3436, December 2002,
            <http://www.rfc-editor.org/info/rfc3436>.
 [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
            Transport Layer Security (DTLS) for Stream Control
            Transmission Protocol (SCTP)", RFC 6083, January 2011,
            <http://www.rfc-editor.org/info/rfc6083>.
 [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
            Yasevich, "Sockets API Extensions for the Stream Control
            Transmission Protocol (SCTP)", RFC 6458, December 2011,
            <http://www.rfc-editor.org/info/rfc6458>.

Tuexen, et al. Standards Track [Page 9] RFC 7496 Additional PR-SCTP Policies April 2015

 [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
            "Specification of the IP Flow Information Export (IPFIX)
            Protocol for the Exchange of Flow Information", STD 77,
            RFC 7011, September 2013,
            <http://www.rfc-editor.org/info/rfc7011>.
 [DATA-CHAN]
            Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
            Channels", Work in Progress, draft-ietf-rtcweb-data-
            channel-13, January 2015.
 [IEEE.1003-1G.1997]
            IEEE, "Protocol Independent Interfaces", IEEE Standard
            1003.1G, March 1997.

Acknowledgments

 The authors wish to thank Benoit Claise, Spencer Dawkins, Gorry
 Fairhurst, Stephen Farrell, Barry Leiba, Karen Egede Nielsen,
 Ka-Cheong Poon, Dan Romascanu, Irene Ruengeler, Jamal Hadi Salim,
 Joseph Salowey, Brian Trammell, and Vlad Yasevich for their
 invaluable comments.

Tuexen, et al. Standards Track [Page 10] RFC 7496 Additional PR-SCTP Policies April 2015

Authors' Addresses

 Michael Tuexen
 Muenster University of Applied Sciences
 Stegerwaldstrasse 39
 48565 Steinfurt
 Germany
 EMail: tuexen@fh-muenster.de
 Robin Seggelmann
 Metafinanz Informationssysteme GmbH
 Leopoldstrasse 146
 80804 Muenchen
 Germany
 EMail: rfc@robin-seggelmann.com
 Randall R. Stewart
 Netflix, Inc.
 Chapin, SC  29036
 United States
 EMail: randall@lakerest.net
 Salvatore Loreto
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 EMail: Salvatore.Loreto@ericsson.com

Tuexen, et al. Standards Track [Page 11]

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